This invention relates to a new gas phase polymerization process using liquid in an otherwise gas-phase process.
The discovery of gas-phase fluidized bed and stirred reactor processes for the production of polymers, especially polyolefin polymers, made it possible to produce a wide variety of new polymers with highly desirable and improved properties. These gas-phase processes, especially the gas fluidized bed process, provided a means for producing polymers with a drastic reduction in capital investment expense and dramatic savings in energy usage and operating costs as compared to other then conventional polymerization processes.
In a conventional gas fluidized bed process a gaseous stream containing one or more monomers is passed into a fluidized bed reactor containing a bed of growing polymer particles in a polymerization zone, while continuously or intermittently introducing a polymerization catalyst into the polymerization zone. The desired polymer product is withdrawn from the polymerization zone, degassed, stabilized and packaged for shipment, all by well known techniques. Most polymerization reactions, e.g., polymerization of olefins, are exothermic, and substantial heat is generated in the polymerization zone which must be removed to prevent the polymer particles from overheating and fusing together. This is accomplished by continuously removing unreacted hot gases from the polymerization zone and replacing them with cooler gases. The hot gases removed from the polymerization zone are compressed, cooled in a heat exchanger, supplemented by additional amounts of monomer to replace monomer polymerized and removed from the reaction zone and then recycled into the bottom of the reactor. Cooling of the recycled gases is accomplished in one or more heat exchanger stages. The sequence of compression and cooling is a matter of design choice but it is usually preferable to provide for compression of the hot gases prior to cooling. The rate of gas flow into and through the reactor is maintained at a level such that the bed of polymer particles is maintained in a fluidized condition. The production of polymer in a stirred bed reactor is very similar, differing primarily in the use of mechanical stirring means to assist an upwardly flowing stream of gases in maintaining the polymer bed in a fluidized condition.
Conventional gas phase fluidized bed resin production is very well known in the art as shown, for example, by the disclosures appearing in U.S. Pat. Nos. 4,379,758; 4,383,096 and 4,876,320, which are incorporated herein by reference.
The production of polymeric substances in gas phase stirred reactors is also well known in the art as exemplified by the process and equipment descriptions appearing in U.S. Pat. No. 3,256,263.
For many years it was erroneously believed that to allow liquid of any kind to enter into the polymerization region of a gas phase reactor would inevitably lead to agglomeration of resin particles, formation of large polymer chunks and ultimately complete reactor shut-down. This concern caused gas phase polymer producers to carefully avoid cooling the recycle gas stream entering the reactor to a temperature below the condensation temperature of any of the monomers employed in the polymerization reaction.
Comonomers such as hexene-1,4-methyl pentene and octene-1, are particularly valuable for producing ethylene copolymers. These higher alpha olefins have relatively high condensation temperatures. Due to the apprehension that liquid monomers in the polymerization zone would lead to agglomeration, chunking and ultimately shut down the reactor, production rates which depend upon the rate at which heat is removed from the polymerization zone, were severely constrained by the perceived need to maintain the temperature of the cycle gas stream entering the reactor at temperature safely above the condensation temperature of the highest boiling monomer present in the cycle gas stream.
Even in the case of polymerization reactions conducted in fluidized, stirred reactors, care was exercised to maintain the resin bed temperature above the condensation temperature of the recycle gas stream components.
To maximize heat removal it was not unusual to spray or inject liquid into or onto the polymer bed where it would immediately flash into a gaseous state by exposure to the hotter recycle gas stream. A limited amount of additional cooling was achieved by this technique by the Joule-Thompson effect but without ever cooling the recycle gas stream to a level where condensation might occur. This approach typically involved the laborious and energy wasting approach of separately cooling a portion of the cycle gas stream to obtain liquid monomer for storage and subsequent separate introduction into or onto the polymerization bed. Examples of this procedure are found in U.S. Pat. Nos. 3,254,070; 3,300,457; 3,652,627 and 4,012,573.
It was discovered later, contrary to the long held belief that the presence of liquid in the cycle gas stream would lead to agglomeration and reactor shut-down, that it is indeed possible to cool the entire cycle gas stream to a temperature where condensation of significant amounts of monomer would occur without the expected dire results when these liquids were introduced into the reactor substantially in temperature equilibrium with the recycle gas stream. Cooling the entire cycle gas stream produces a two-phase gas-liquid mixture in temperature equilibrium with each other so that the liquid contained in the gas stream does not immediately flash into vapor. Instead, a substantially greater amount of cooling than previously thought possible takes place because the total mass of both gas and liquid enters the polymerization zone at a temperature substantially lower than the polymerization zone. This process led to substantial improvements in the yield of polymers produced in the gas phase, especially where comonomers which can condense at the temperatures of the polymerization zone, are used. This procedure, commonly referred to as xe2x80x9ccondensing modexe2x80x9d operation, is described in detail in U.S. Pat. Nos. 4,543,399 and 4,688,790 which are incorporated by reference.
In condensing mode operation, the two-phase gasliquid mixture entering the polymerization zone is heated quite rapidly and is completely vaporized within very short distance after entry into the polymerization zone. Even in the largest commercial reactors, soon after entry into the polymerization zone all liquid has been vaporized and the temperature of the then totally gaseous cycle gas stream raised, by the exothermic nature of the polymerization reaction. The ability to operate a gas phase reactor in condensing mode was believed possible due to the rapid heating of the two-phase gas liquid stream entering the reactor coupled with efficient constant back mixing of the fluidized bed leaving no liquid present in the polymer bed more than a short distance above the entry level of the two-phase gasliquid recycle stream.
Commercial polymerization operations have used for years relatively high levels of condensate in the recycle streams, in many instances in excess of 20 weight percent liquid was contained in the recycle stream but always above, the dew point for components in the polymerization zone to assure quick volatilization of the liquid.
While fluidized bed polymerization processes have found particular advantage in the manufacture of polyolefins, the types of polymerization catalysts have been limited to those which are operable in the gas phase. Consequently, catalysts that exhibit activity in solution phase reactions and those which operate by ionic or free radical mechanisms are typically not suitable for in gas phase polymerization processes.
We have now found that in gas or vapor phase polymerization processes (the terms are used interchangeably in the art and in this specification), by providing at least one component in the polymerization zone, which component is capable of being liquid under the temperature, pressure and its concentration in the polymerization zone (herein referred to as xe2x80x9cLiquid Componentxe2x80x9d), the polymerization process is enhanced. The concentration of the Liquid Component is maintained in the process of this invention, below that which unduly adversely affects the ability of the polymer bed to be fluidized and remain in the gaseous or vapor phase.
While not limited to any particular type or kind of polymerization reaction, this invention is particularly well suited to olefin polymerization reactions involving homopolymerization and copolymerization of relatively high boiling monomers.
Examples of higher boiling monomers capable of undergoing olefinic polymerization reactions are the following:
A. higher molecular weight alpha olefins such as decene-1, dodecene-1 etc. and styrene.
B. dienes such as hexadiene, vinyl cyclohexene, dicyclopentadiene, butadiene, isobutylene, isoprene, ethylidene norbornene and the like.
C. polar vinyl monomers such as acrylonitrile, maleic acid esters, vinyl acetate, acrylate esters, methacrylate esters, vinyl trialkyl silanes and the like.
These higher boiling monomers can be homopolymerized in accordance with this invention with the use of an inert gas as the gaseous component of the two phase gas-liquid mixture cycled through the reactor. Suitable inert materials for this purpose include nitrogen and saturated hydrocarbons which remain gaseous at a temperature below the temperature selected to be maintained in the polymerization zone.
The higher boiling monomers can also be copolymerized with one or more lower boiling monomers such as ethylene, propylene and butene, as well as with other higher boiling monomers such as those mentioned above, the only requirement being that there be a sufficient difference in the condensation temperatures of the higher boiling monomer and at least one lower boiling monomer or inert substance as will allow enough gas to be present in the cycle gas stream to permit practical, steady state, continuous operation.
In accordance with our invention the higher boiling monomers can be directly introduced into the polymerization zone or carried into the polymerization zone as with the recycle gas stream.
Enhancements that may be achieved in accordance with this invention include one or more of the following: increases in production rate; improved catalyst productivity (particularly for catalysts that tend to deactivate, or exhibit accelerated rates of deactivation, with increasing temperature) leading to reduced catalyst residues and lower catalyst costs; reduction in localized regions of higher temperature (xe2x80x9chot spotsxe2x80x9d) in the polymerization bed, facilitated operation control particularly for maintenance of desired temperatures; practical ability to operate at temperatures closer to the fusion temperature of the polymer particles being produced since the Liquid Component provides better heat control; improved operation through reduction in the generation of static; improved ability to make sticky polymers; reduction in the risk of fusion of polymer upon emergency shut-down of the reactor; improved ability to operate at higher bed density ratios; improved efficiency in conversion of monomers to polymers through the reduction of fines exiting the polymerization zone and reduced fouling within the reaction system of the type caused by the presence of fines; enhanced ability to control comonomer incorporation in a copolymer; ability to use catalysts that otherwise would not be attractive for fluid bed polymerization processes such as ionic and free radical catalysts; enhancements in the use of solution catalysts for gas phase polymerizations; an ability to enhance the polymer product through morphology control and incorporation of other polymers and additives; an ability to achieve more uniform product properties via more uniform temperatures between different particles and within polymer particles during polymerization, through morphology control, and through incorporation of other polymers and additives.
The processes of this invention involve the production of polymer by the reaction, usually exothermic, of one or more monomers in a fluidized bed reaction vessel having a polymerization zone containing a bed of growing polymer particles. The fluidized bed may be maintained solely by the upwardly flowing gases or may be a stirred bed process. Stirred bed processes are those in which the stirrer cooperates with an upwardly directed flow of gases to assist in the fluidization of the polymer particles. In general, the processes comprise:
a) continuously or intermittently introducing the one or more monomers into said polymerization zone;
b) continuously or intermittently introducing at least one polymerization catalyst into said polymerization zone;
c) continuously or intermittently withdrawing polymer product from said polymerization zone;
d) continuously withdrawing gases from the polymerization zone, compressing and cooling said gases for recycle to the polymerization zone; and
e) continuously maintaining sufficient gas flow through the polymerization zone to maintain the bed in a fluidized state, said gas flow comprising recycle of at least a portion of the gases withdrawn from the polymerization zone, wherein at least one Liquid Component is provided in the polymerization zone. A bed is fluidized where substantially all the particles in the bed are suspended in the gas and the particles behave like a fluid.
In one preferred embodiment of the invention, the Liquid Component is provided in the polymerization zone in an amount greater than that which can be absorbed by the polymer particles, and the amount of the Liquid Component that is in excess of the amount that can be absorbed by the polymer particles, is capable of being in the liquid phase throughout the polymerization zone. Preferably, the Liquid Component is provided in an amount of at least 1 percent by weight based upon the weight of the bed.
In another preferred embodiment, the Liquid Component is provided throughout the polymerization zone in liquid and gaseous phases, and is present in the gases in an amount sufficient that substantially no net vaporization of liquid phase Liquid Component into the gaseous medium occurs in the polymerization zone. Thus, the amount of Liquid Component in the liquid phase in the polymerization zone is substantially constant under steady state operating conditions.
In another preferred embodiment, sufficient liquid component is provided to enable the bed to be reduced in height to a level below that which could be obtained by substantially the same process but having the liquid component replaced with an inert, non-condensable gas. The liquid component in the gas and on or in the polymer particles can significantly change the fluidization properties such that this turn-down can be achieved. The turn down enables transitions from one catalyst or polymer to another to be achieved rapidly and with the production of minimal off-grade polymer.
In another preferred embodiment, the Liquid Component permits the polymerization zone to be operated at a high bed density ratio (xe2x80x9cFBDxe2x80x9d) (settled bed density divided by fluidized bed density). In this embodiment, the Liquid Component is provided in the polymerization zone in an amount sufficient to increase the bed density above that achieved by a similar process but in which the liquid component is replaced with an inert, non-condensable gas. Advantageously, the Liquid Component is provided in an amount such that the bed density is increased by an amount of at least about 10, preferably at least about 20, percent of the difference between 1.0 and FBDS wherein FBDS is the bed density achieved using the inert, noncondensable gas in place of the liquid component.
In another preferred embodiment, the at least one Liquid Component is provided in an amount such that the gases withdrawn from the polymerization zone contain at least a portion of the Liquid Component in the liquid phase.
In another preferred embodiment, the at least one Liquid Component is provided in an amount sufficient to substantially eliminate the generation of static in the polymerization zone.
In another preferred embodiment, the at least one Liquid Component is provided in an amount sufficient to substantially eliminate or reduce the presence of fines in the gases withdrawn from the polymerization zone. Preferably, the fines in the gases withdrawn from the polymerization zone are reduced by at least about 50 weight percent as compared to those in a similar process but having the Liquid Component replaced with inert, non-condensable gas. Often fines having a major dimension of less than about 75 microns, and preferably less than about 100 microns, are substantially eliminated from the gases leaving the polymerization zone as compared to a similar process but not containing the Liquid Component.
Another preferred embodiment of this invention relates to producing polymer particles that are sticky at the temperature of the polymerization zone. In this aspect, the at least one Liquid Component is provided in an amount sufficient to substantially prevent undue agglomeration of polymer particles in the polymerization zone. Undue agglomeration results in the formation of particles that are so large as to disrupt the fluidization of the bed or cause fouling of the reaction vessel walls or are larger than desired for polymer product. Generally, unduly large agglomerates have a major dimension greater than about 5, sometimes greater than about 2, centimeters. In this feature of the invention, the Liquid Component preferably has a limited solubilityin the polymer and the Liquid Component is provided in an amount in excess of that which can be dissolved in the polymer in the polymerization zone.
Another preferred embodiment of the invention relates to the production of polymer, wherein upon loss of the gas flow to maintain the bed fluidized and the polymer particles settle in the presence of monomer, the exothermic polymerization reaction can continue and increase the temperature of the polymer particles to a temperature at which the particles stick together or fuse. In this feature, the at least one Liquid Component is provided in an amount sufficient to delay or prevent an increase in the temperature within the settled polymer bed to a temperature at which the unfluidized particles fuse. If the undue temperature rise is delayed, the delay should be for a time sufficient to introduce a kill agent to stop the polymerization, e.g., for at least about 5 minutes, preferably, at least about 10 minutes. Kill agents are well known in the art. Preferably, the Liquid Component is provided in an amount sufficient to prevent localized fused regions greater than about 30 centimeters in major dimension, from forming.
Beyond the reduced risk of polymer fusion one can take further advantage of this feature of the invention by increasing the polymerization zone temperature closer to the particle fusing temperature. In commercial fluid bed operations a healthy temperature margin is often left between the polymerization zone temperature and the polymer fusing temperature to avoid the risk of fusing. Increasing the polymerization zone temperature enables a greater polymer production rate out of existing or new equipment than would be obtained at lower temperatures. This occurs due to the greater heat removal capacity due to a greater temperature difference between the recycle gas stream and the cooling water temperature. Furthermore this enables catalysts to be operated at higher temperatures than were possible before without undue risk of polymer fusion. Some catalysts will have higher productivity or other performance advantages and/or make better products in the newly accessible temperature region.
In another preferred embodiment of the invention, the at least one Liquid Component is provided in an amount sufficient to enhance the production rate of polymer, even at the same average bulk temperature in the polymerization zone. Preferably, the observed increase in production rate is at least about 5 percent as compared to that provided by substantially the same process but replacing the at least one Liquid Component with an inert, non-condensing gas, wherein the dew point of said at least one Liquid Component under the conditions of the polymerization zone is within about 2xc2x0 C. of the average bulk temperature of the polymerization zone.
Another preferred embodiment of this invention relates to processes deleteriously high localized temperatures can be generated due to the exothermic nature of the polymerization reaction. These temperatures may, for example, tend to deactivate the catalyst or accelerate the polymerization reaction to a level where the heat removal capacities are insufficient to control temperature. In this feature, the at least one Liquid Component is provided in an amount sufficient to protect the catalyst from deleteriously high, localized temperatures. Hot spots can be avoided in that heat generated by the polymerization is absorbed by the mass of Liquid Component present and, if the Liquid Component is capable of being vaporized, is consumed in the vaporization of at least a portion of the Liquid Component in the region. Some or substantially all the Liquid Component that is vaporized may condense in the cooler sections of the polymerization zone or outside the polymerization zone. In a preferred embodiment, where highly active spots exist on the catalyst and localized generation of heat increases, the Liquid Component is vaporized to prevent unduly deleterious high temperatures from being achieved. In some instances, where localized regions of heat are generated that cause growing polymer particles to undergo undue agglomeration, the volume increase associated with the vaporization of Liquid Component may physically break apart the agglomerate and facilitate cooling of the region by the fluidizing gases.
Another preferred embodiment of this invention relates to processes for producing copolymer by the reaction of two or more monomers. The monomers may be continuously or intermittently introduced simultaneously or separately into the polymerization zone. The at least one Liquid Component, where sorbed on and in the growing polymer particles, is capable of affecting the rate of incorporation into the polymer of at least one monomer as compared to at least one other monomer. For instance, the Liquid Component sorbed on the growing particles may be rich in one or more of the monomers as compared to at least one other of the monomers as a means to promote preferential monomer incorporation. By way of example, one or more monomers may have preferential solubility in the Liquid Component and thus affect comonomer concentration at the catalytic site and its relative rate of incorporation into the polymer on a continuous basis. In one embodiment, the Liquid Component may become depleted of this monomer and thus the composition of the polymer particle may change during the time that it is in the polymerization zone, and a given polymer chain may have differing amounts of comonomer incorporation over its length. In a preferred embodiment of this aspect of the invention, ethylene is a monomer and the at least one other monomer has a reactive olefinic bond and from 3 to 36 carbon atoms.
Another preferred embodiment of this invention facilitates or enables the use of polymerization catalysts that are solution, ionic or free-radical catalysts in a gas phase process. In this feature, the at least one Liquid Component is in contact with the catalyst in an amount sufficient for the catalyst to effect the polymerization. Thus, the Liquid Component provides the media to enable the catalyst to function or function more effectively.